1. Field of the Invention
The present invention relates to alumina-based ceramics whose surface is modified into a high toughness layer and to a method for manufacturing the same.
2. Description of the Prior Art
Alumina-based ceramics are widely used as ceramic components in modern electronic and machine industries, for example IC boards, bearings, cutting tool, etc. Like other ceramic materials, alumina-based ceramics are greatly limited in their applications by their susceptibility to brittle fracture. When flaws are present on the surfaces, their strength degrades considerably. Then, brittle fracture may catastrophically occur from the flaws, bringing about the poor reliability of the components. This is a great obstacle to expand their practical applications.
For decades, a number of studies have been conducted in order to control the microstructure for strengthening alumina-based ceramics. As exemplified by reduction of grain sizes to decrease the dimension of the flaws, by addition of a second phase, by quenching and by substitution of Al2O3 with Cr2O3 at the surface to induce compressive stresses, these strengthening methods were generally associated with an improvement in the strength of the material (E. Dxc3x6rre and H. Hxc3xcbner, Alumina: Processing, Properties, and Applications; pp. 74-192, Springner-Verlag Berlin, Heidelberg, 1984, 1). In order to prevent the catastrophic fracture attributable to the inherent high brittleness of ceramics and the decrease in reliability resulting from it, however, not only high strength, but also high toughness is required. In other words, a high flaw tolerance is required in order that the ceramics do not undergo a strength decrease even if flaws exist on the surface.
When the toughening of monolithic ceramics are related to the bridging and pull-out of grains, possible ways for improving its toughness are an increase in the number of bridging grains and an enlargement of the wake zone. In this regard, there is a need to control the microstructure of the material to have large number of bridging grains, to increase the aspect ratio of the grains, or to introduce residual stresses with a grain boundary strength being sufficiently decreased.
The above-mentioned directions of microstrucutre control for improving fracture toughness in long-crack regions, however, are reported to suffer from the drawbacks of strength reduction due to large grain size and short-crack toughness decrease due to weakened grain boundary strength (N. P. Padture, C. J. Evans, H. H. K. Xu and B. R. Lawn, Enhanced Machinability of Silicon Carbide via Microstructural Design, J. Am. Ceram. Soc., 78[1] 215-17 (1995)). These drawbacks also give rise to fatal problems, including decreases in wear resistance and fatigue properties, in association with short cracks.
To meet all of the physical properties for structural parts, such as strength, toughness, wear resistance, fatigue properties, etc., materials which are individually superior in one of the properties are often made to be composite. Conventional composites have microstructures with each constituents being homogeneously distributed. Thus, the physical properties of a composite have a tendency to change in proportion to the relative amounts of constituent materials. Such a composite material, however, cannot satisfy both long-crack properties such as toughness and short-crack properties such as wear resistance and fatigue properties, simultaneously.
Therefore, there remains a need for an improved composite material that is suitable for use in structural parts, which require excellent and balanced physical properties.
Leading to the present invention, the extensive and intensive research on a composite material, repeated by the present inventors aiming to solve the problems encountered in prior arts, resulted in the finding that, when being formed into a layer structure in which materials with desired physical properties are properly distributed on and inside the surface or being provided with a modified surface or a coating layer of high short-crack toughness, a material with high long-crack toughness could show appropriate strength, toughness, wear resistance and fatigue properties which meet the conditions for specialized uses. When designing the layer structure, account must be taken of the interlayer stress, elastic/plastic mismatches, the layer thickness, and the grain boundary strength between layers. Unless these conditions are optimized, the resulting composite material may be rather weakened.
Therefore, it is an object of the present invention to provide alumina-based ceramics with a modified surface which has little difference in the interlayer stress and elastic/plastic mismatches.
It is another object of the present invention to provide a method for manufacturing a surface-modified alumina-based ceramics, which is simple and can be easily applied to conventional manufacturing method of alumina-based ceramics.
In accordance with the present invention, there is provided a method for manufacturing a surface-modified alumina-based ceramics, comprising the steps of: sintering an iron oxide-containing alumina powder compacts in an atmosphere of a relatively low oxygen partial pressure; and annealing the sintered body in an atmosphere higher in oxygen partial pressure than the sintering atmosphere, whereby the alumina-based ceramics has a grain-boundary migration layer on its surface.
In another aspect of the present invention, there is provided monolithic or composite ceramics, which is surface modified by chemically induced grain boundary migration.